The invention relates to structures with a multiply fabric reinforcement having filler threads extending between the plies and to a method of producing such structures.
The German utility model 7827000 describes a multiply fabric with a double ply configuration of this type. The space between the plies or layers is either subsequently filled with a foaming material or it is so woven that a system of filler threads is incorporated in the multiply fabric, such filler threads then functioning also as spacer threads. The plies themselves are joined together by binder threads.
The objects of the present invention is to devise a structure of the initially mentioned type on a fabric basis which, while having a high strength, is also low in weight.
According to a principal aspect of the invention, the structure includes at least two fabric plies formed of aramid fibers, carbon fibers, ceramic fibers or fiberglass fibers, filling threads made of polypropylene, polyamide or carbon fibers and which extend in a warp direction between the fabric plies, and a cured resin contained within and between the plies but such that the filling threads have resin-free areas therein.
Owing to this design the invention provides a high-strength, sheet-like structure with a low weight. For products of this type there is naturally a wide range of applications. The high strength achieved by the structure, which is practically to be considered as an organic material having the properties of metal plate, results from the use of technical yarns such as aramide, carbon and ceramic fibers. In particular, a blend of such high performance yarns is conceivable and may be adapted to the respective application.
As regards the filling fiber system, polypropylene or even polyamide is found to be highly satisfactory. In such a case the feature of the low weight is particularly advantageous. The specific gravity is 0.91 g per cc. Added to this the material is very supple. The filaments forming part of a group of threads or sliver occlude the greater part of the cross section of the filling threads to keep out the curing resin material. Only the outer surface part of the material is wetted so that there are still continuous resin-free zones formed by the filling threads (which are pressed together). The latter are incorporated as weft threads.
As regards the fabric structure, there is on the whole a good impregnation with the resin. Furthermore in the case of a structure of component there is the advantage that the multiply fabric reinforcement is made of a technical-quality yarn, more especially fiberglass, and that the filling threads incorporated as weft threads are a group of threads of carbon fiber. This leads to an optimum overall stability and strength, more especially since substantial accumulations of such fiber are possible. Their relatively ropy structure also offers advantageous as regards molding inasfar as the owing to relative shift of the filaments the package of filaments allows satisfactory shaping, it counteracting the tendency to return to the original form which is otherwise to be met with and is due to the friction between the relatively displaced filaments. It is specifically this property which is an advantage for the construction of containers or for instance the fuselage or hull of airplanes and ships. A further advantage in a connection with the best possible impregnation in connection with a "porous" fraction which is nevertheless large is that the filaments of the group of threads of polypropylene or polyamide is irregularly crimped and in the case of a further reduction in the number of filaments this is responsible for a thread group configuration which has an even larger amount of cavity. The fixation of the filling system to the plies forming the multiply fabric is caused by binding threads extending in the warp direction. In this respect there is the further feature that the upper and lower plies are woven using flat fibers. This leads to a thin sheet structure with a plain weave like webbing facing all directions of load with its broad side. There is the further feature of the invention in this connection that the breadth of the flat threads is to be approximately half the breadth of the filling threads.
An advantageous method for the manufacture of a premolding is such that, in order to achieve different thicknesses of the premolding, shims are placed in the mold. Such shims may be in the form of rails in order to create grooves, etc. Members with a more sheet-like form could also be possible, as for instance in order to produce a zone set back from the main surface for the mounting in a protected manner of a label plate with indications of the material used or of the origin, etc. It is an advantage if the shim elements are metallic. Such elements stamped from sheet metal have the advantage of being able to be changed in position. Furthermore they provide a wide range of variation in thickness. In fact, a cushion-like filling thread system makes possible a sufficient displacement in the depth direction even although the rear side is flat. A surprising effect was discovered in this respect inasfar as although the surface structure is rough and fabric-like, where the shims have been placed there is a completely smooth surface (comparable with that produced by rubbing).
If suitably transparent basic material is used, this may in some cases even lead to a form of transparency like glass. This in turn permits the use of the structure as part of photoelectric detecting equipment. A particularly satisfactory combination of materials is furthermore one in which the warp threads of the multiply fabric are fiberglass and the weft threads are carbon fiber. In this respect a good ratio between such materials is one in which the filling threads are present as a multiple of the amount of the warp threads, such multiple being between 5 and 25 times the amount of the warp threads. It is furthermore an advantage if the receiving chamber of the filling threads is formed by the binding structure having the weft threads and the ply structure which here is free of weft threads. This makes possible specially free access for the incorporation of the resin. The multiply fabric may thus have a substantial thickness, that is to say practically 8 or 10 plies. It is furthermore possible for the binding warp threads of the multiply fabric to meander and for the ply warp threads to intersect with the binding warp threads with the formation of the receiving chambers for the filling threads with the result that an extremely stable sandwich configuration is produced, which nevertheless has the necessary degree of elasticity as regards shaping the component.
Lastly in accordance with a further possible feature of the invention the binding warp threads have at least one binding intermediate weft thread between the weft threads of the fabric plies. This leads to a chamber wall, free of any interruptions, in the form of a woven ring. It is furthermore possible for the ply warp threads of the fabric plies to have at least one ply intermediate weft thread between the points of intersection with the binding warp threads. A feature of this type is applicable if large receiving chambers, which are elongated in the warp direction, are to be produced.
The invention will now be described in more detail with reference to working embodiments as shown in the drawings.